This invention relates to a substrate for an information recording medium for use as a recording medium for an information processing apparatus, an information recording medium using the substrate, and a method of producing the substrate.
A magnetic disk is known as one of information recording media. The magnetic disk comprises a substrate and a thin film such as a magnetic layer formed thereon. As the substrate, use has been made of an aluminum substrate or a glass substrate. In recent years, in response to the demand for high-density recording, the glass substrate is used at an increasing ratio because a gap between a magnetic head (which is operable as a recording and/or reproducing head) and the magnetic disk can be small as compared with the aluminum substrate.
Generally, the glass substrate is produced through chemical strengthening in order to increase the strength so that the glass substrate is resistant against a shock when loaded into a magnetic disk drive. Alternatively, the surface of the glass substrate is heat treated to be crystallized so that a crystallized substrate improved in strength is produced. In addition, in order to lower a flying height of the magnetic head as low as possible, the surface of the glass substrate is polished with high precision. Thus, high-density recording is realized.
Not only the glass substrate, the magnetic head is also developed from a thin film head to a magnetoresistive head (MR head), further to a giant (large-sized) magnetoresistive head (GMR head) so as to meet the high-density recording.
However, there arises a problem that, even if the surface roughness Rmax (which is defined as a maximum height representative of a difference between a highest point and a lowest point) or Ra (which is representative of a center-line-mean roughness) is reduced by high-precision polishing, the flying height of the magnetic head can not be lowered. The present inventors investigated the cause to find out that a microwaviness present on the surface of the substrate is responsible and that the microwaviness also affects a modulation of the magnetic disk.
Japanese Unexamined Patent Publication (JP-A) No. H08-147688 proposes a glass substrate for a magnetic disk, in which a surface waviness (Wa) and the surface roughness (Ra) satisfy the relationship Wa/Ra≦0.5 in order to reduce a glide height of the magnetic disk.
However, measurement of the surface waviness described in the above-mentioned publication uses a tracer-type surface roughness tester (Tencor) and is carried out over the length on the order of 100 μm along a straight line in a radial direction of the substrate. Thus, the measurement is performed only in a restricted local area and in a single direction. Thus, the above-mentioned measurement does not fully reflect the surface waviness of the whole substrate.